The formation efficiency of lead-acid batteries depends to a great extent on the positive plate, in particular, to the extent of conversion of lead monoxide (PbO) to lead dioxide (PbO.sub.2) in the active positive material. Positive plates, which are pasted lead current collectors, are generally more difficult to form than negative plates. The high electrical potential required for formation appears to be related to the transformation of non-conductive paste materials to PbO.sub.2. A low formation efficiency of positive plates requires a high formation charge.
Inefficient charging also leads to deficiencies in the resulting batteries assembled with such plates. Typically, the rated capacity (performance) of the battery is low, requiring additional cycling to reach specific performance values. The battery also tends to self-discharge even over relatively short periods of time.
A plate for a lead-acid battery may be prepared by preparing a paste mixture of lead oxide, sulfuric acid and water. The lead oxide reacts with the sulfuric acid to form mono-, di-, tri- or tetrabasic lead sulfate(s). Dry additives such as fiber and expander may be added. The plate is then applied to a lead grip. The pasted plates are next typically cured for many hours under elevated temperature and humidity to oxidize free lead (if any) and adjust the crystal structure of the plate, e.g., convert tribasic to tetrabasic lead sulfate. Depending on the mixing and curing procedures used, the lead paste elements typically consist of lead monoxide, lead hydroxide, tribasic lead sulfate and tetrabasic lead sulfate. The basic lead sulfates provide structure in the plate which is necessary for good battery performance.
After curing, the positive plates are assembled into batteries and electrochemically formed by passage of current to convert the lead sulfate or basic lead sulfate(s) to lead dioxide. Several attempts have been made to improve the conductivity of the paste and thus improve the formation efficiency. For example, it is known to apply a hydrogen peroxide solution to the surfaces of battery plates by painting, dipping or spraying. See Orsino U.S. Pat. No. 2,658,097 issued Nov. 3, 1953. Ozone has been used to treat battery plates, as described in Mahato U.S. Pat. No. 4,656,706, issued Apr. 14, 1987.
In particular, reacting a persulate (S.sub.2 O.sub.8.sup.2-), also known as peroxydisulfate or peroxodisulfate, with the pasted plate has been suggested. Persulfate reacts with lead monoxide and water to form lead dioxide. U.S. Pat. No. 2,159,226, issued May 23, 1939 to Reid, discloses the use of persulfate to improve the formation efficiency of lead battery plates. The plates are dipped into an ammonium persulfate solution, or in persulfate is added directly to the battery paste. The persulfate solution consists of ammonium persulfate in dilute sulfuric acid. According to Reid, the persulfate pickling bath need not be an acid solution, and the persulfate may be added to any desired pickling solution, such as neutral aqueous solutions of ammonium sulfate or sodium sulfate. Pickling generally refers to the process of forming lead sulfate(s) by the reaction of lead oxide with sulfuric acid. Since acid is commonly used for this purpose, a pickling bath is not alkaline.
Other variations of persulfate positive plate treatments have been proposed. Like the original Reid process, all of these include conducting the persulfate reaction in an acidic environment. For example, U.S. Pat. No. 3,398,024, issued Aug. 20, 1968 to Barnes and Armstrong, discloses a method for obtaining better adhesion of the paste to the lead grid by dipping the grid prior to pasting in a persulfate or perborate solution, and then pasting the grid while it is still wet. Belgian Patent No. 723,018, published Oct. 28, 1968, describes another variation of the persulfate process involving paste preparation by mixing lead oxide and a small amount potassium persulfate with water until the lead oxide is dispersed uniformly throughout the mixture, and then applying the paste mixture to an electrically conductive grid support, partially drying the plate, and then dipping it in sulfuric acid. Japanese Patent Publication No. 62-145664, published in 1985, describes a process of dipping pasted grids into an acidic ammonium persulfate solution. Spanish Patent Publication No. 8801559, published May 10, 1988, similarly discloses immersing pasted plates, prior to formation, in an acidic persulfate solution, or adding persulfate to the acid of the battery prior to formation.
As to known uses of alkaline persulfate solutions, such solutions have been used to deposit lead dioxide on ceramic substrates which are subsequently converted by electrodeposition to lead dioxide electrodes for use in electrolysis reactions. See, e.g., U.S. Pat. No. 4,008,144, issued Feb. 15, 1977 to Torikai et al., which discloses conversion of water-soluble lead salts deposited in a ceramic substrate to lead dioxide by dipping the substrate in alkaline persulfate solutions.
Neither the dipping method nor the addition of persulfate to the battery paste are effective if free lead is present in the paste, and for this reason neither method is effective for making uncured battery plates containing free lead. Thus, while the prior art has attempted to address the problem of positive plate formation inefficiency, the known persulfate methods have disadvantages.
Treatment with an aqueous alkaline persulfate solution to effect the conversion of lead monoxide to lead dioxide has been proposed as an alternative to process discussed in the foregoing patent to Reid conducted in a neutral or acidic environment. However, such an alkaline solution tends to convert lead sulfate and basic lead sulfates to lead oxide. Such lead sulfates are desirable paste components because they increase plate porosity and improve performance. The present invention addresses this problem and additionally provides a new method for application of a persulfate solution.